1. Field of the Invention
This invention relates generally to the field of cancer therapy and treatment. Further, the invention relates to novel endocrine therapies for treating cancers, and in particular, cancers that express different ratios of progesterone receptor isoforms. Depending on the prevailing isoform expressed, such cancers may become sensitive or resistant to an antiprogestin therapy. The invention particularly provides methods and compositions for overcoming resistance to these progesterone receptor-related endocrine therapies in cancers, and especially in breast cancers. Moreover, the invention relates to screening methods for detecting tumors that express certain progesterone receptor isoforms.
2. Background
Breast cancer is the most frequently diagnosed malignant neoplasia and is a leading cause of cancer death in females worldwide. Breast cancer ranks second overall in cancer mortality (10.9%) and accounts for 23% (1.38 million) of new cancer diagnoses and 14% (458,400) of total cancer deaths (Jemal, et al. 2011). Breast cancer is not a single disease but instead constitutes a spectrum of lesions with distinct cellular origins, somatic changes, and etiologies.
Gene expression studies have divided breast cancer into several categories, including, but not limited to, basal-like, ErbB2-enriched, normal breast-like (adipose tissue gene signature), luminal subtype A, luminal subtype B and claudin-low (Prat, et al. 2010). More than 66% of breast carcinomas express estrogen receptor alpha (ERα) and respond to anti-estrogen therapies.
These carcinomas may also express progesterone receptors (PRs), which are a reliable marker of functional ERs (Kastner, et al. 1990; Petz and Nardulli 2000). Estrogen and progesterone and their respective receptors are widely regarded as playing important roles in the etiology of breast cancers.
Endocrine therapies seeking to block or inhibit the action of estrogen have been known for some time now but the emergence of resistance to such therapies remains a limitation. For example, antiestrogen treatment, such as tamoxifen therapy, remains a central and successful approach in the treatment of this disease but resistance remains a major setback. Most tumors initially respond to antiestrogen therapy, but many will eventually develop resistance (acquired hormone resistance). Moreover, some tumors fail to respond to endocrine treatment from the beginning (constitutive resistance) despite expressing hormone receptors. Much less is known about the role of the progesterone receptor (PR) in cancer etiology, or its role as a viable target for antiprogestin-based therapies in the treatment of cancer.
The PR is a member of the steroid-thyroid hormone-retinoid receptor superfamily of ligand-activated nuclear transcription factors (Evans 1988; Kastner et al. 1990). Upon progesterone binding, which has been shown to be required for the proliferation of mammary glands and mammary carcinomas, the receptor undergoes a series of conformational changes, dimerizes and translocates to the nucleus, where it interacts with specific DNA sequences (Progesterone Response Elements, PREs) in the promoter regions of target genes (Edwards, et al. 1995; Lange et al. 2008). These transcriptional effects may also be mediated by PRE independent actions through protein-protein interactions between the PR and other sequence specific transcription factors (Leonhardt, et al. 2003). The PR, like all transcription factors, localizes to the nuclear compartment. It has also been described to be located in the cytoplasm and at the cell membrane (Bottino, et al. 2011), where it triggers non-genomic or membrane initiated signaling pathways.
Accordingly, progesterone receptors are members of the steroid hormone receptor family which are ligand-activated nuclear transcription factors, which when bound by progesterone, dissociate from chaperone proteins, dimerize, and bind to specific DNA sequences, enhancing transcription of target genes. PR target genes encode a wide range of proteins that control or modulate crucial cellular functions, such as cell growth, apoptosis, transcription, steroid and lipid metabolism (Li and O'Malley 2003).
Two PR isoforms have been described: isoform B (PR-B), which is 933 amino acids long in humans with a molecular weight of 116 kDa, and isoform A (PR-A), which lacks 164 amino acids at the N-terminus but is otherwise identical to isoform B (MW: 94 kDa; see FIG. 1). They are transcribed from two different promoters of the same gene on human chromosome 11 q22-q23 (Kastner et al. 1990) or on chromosome 9 in mice (band 9A1). In mice, the isoforms have a molecular weight of 115 and 83 kDa, respectively (Schneider, et al. 1991).
When PR-A and PR-B are present in equimolar amounts in wild-type PR-positive cells or are transiently co-expressed in PR-negative cells, they dimerize and bind to DNA as three species: A/A and B/B homodimers and A/B heterodimers. Post-transcriptional modifications of the PR include acetylation, sumoylation and ubiquitination (Dressing and Lange 2009; Hagan, et al. 2009), and especially including phosphorylation. Phosphorylation affects the ability of the PRs to interact with the promoters of their target genes and the subsequent transcriptional activation of these genes (Clemm, et al. 2000). Additionally, phosphorylation affects PR subcellular localization and stability and its ability to interact with other proteins (Clemm et al. 2000).
There is also increasing evidence that isoforms PR-A and PR-B have different functions in vitro and in vivo. It has been speculated that differential expression of PR-A and PR-B is critical for an appropriate mammary gland response to progesterone. Indeed, in transgenic mice carrying an excess of PR-A, mammary gland development is characterized by disproportionate lateral ductal branching, whereas transgenic mice overexpressing PR-B show alterations in lobulo-alveolar growth. PR-A null mice, which only express PR-B, exhibit normal mammary gland development, although they show severe reproductive defects, while PR-B null mice show impaired branching morphogenesis. Taken together, this suggests that PR-A and PR-B have different functions in different tissues and that the described alterations are related to their relative expression ratios.
It has been further observed that PR-A is often over expressed as compared to PR-B (Graham, et al. 2005; Graham, et al. 1995) in breast tissue. In addition, higher molar amounts of PR-A to PR-B have been associated with poorer outcome in patients undergoing hormonal therapy (Hopp, et al. 2004) and even resistance to hormone treatments. Therefore, the PR isoform ratio may be important in breast cancer prognosis and therapeutic decisions, and a clear understanding of the role the different isoforms play in cancer development and hormone resistance will be crucial in the development of hormone anticancer therapies, and in particular, in the use of antiprogestins in treating cancer.
Methods and compositions which would enable the improved use of hormone anticancer therapies, and in particular, the use of antiprogestins for the treatment of cancers, and in particular, breast cancers, which are or may become resistant to such therapies, would be an important advance in the art.